Multi-component mixing and dispensing systems are commonly used to spray or otherwise dispense highly reactive fluids that are stored in separate containers and that flow under pressure through separate fluid lines to a dispensing gun having an on/off valve, and having a mixing nozzle (often a disposable piece) downstream of the on/off valve. Because the fluids begin a chemical reaction as soon as they come into contact with one another, the on/off valve must generally be turned on with sufficient frequency so that the mixed fluids do not have sufficient time to harden to a point where they would block flow through the nozzle. However, in addition to the risk that the on/off valve will inadvertently be turned off for too much time, there is a risk that a first of the component fluids could flow into the fluid line that is intended to contain only a second of the component fluids, which would result in a chemical reaction taking place in that fluid line and entirely blocking it. This condition can occur quickly and may result in significant down-time for the dispensing gun to be at least partially disassembled and manually cleaned out with tools and solvents, before re-assembly and re-use.